The present invention relates to a speaker system having a bass-reflex cabinet.
In the speaker system, the formation of the cabinet affects the reproduction of low frequency sound. A bass-reflex cabinet is provided with a port formed in a front panel of a closed box. The sound rearwardly radiated in the cabinet is discharged from the port for increasing the low frequency characteristic.
FIG. 11 shows a conventional bass-reflex cabinet. A cabinet 1 comprises a speaker unit 2 secured to a front panel 1A of the cabinet 1. A bass-reflex duct 3 having a port 3a is provided on the front panel 1A at a lower portion of the speaker unit.
When the speaker unit 2 emits sound, a part of the sound is radiated in the rearward direction in a cavity la of the cabinet 1 and enters the duct 3. The sound is emitted from the port 3a and corrected into the same phase as the sound forwardly radiated from the speaker unit 2. Thus, the low sound characteristic is increased.
The cavity 1a of the cabinet 1 has compliance C0 and the duct 3 has mass m0.
A dividing network comprising a combination of a low-pass filter and a high-pass filter is used for deriving a frequency band corresponding to a particular speaker system. The dividing network is provided between an amplifier and the speaker system so as to divide an audio-frequency output of the amplifier into high, middle and low bands of frequencies. In order to increase the low frequency characteristic, the middle and high frequency bands are cut to extract the low frequency range which is applied to the bass-reflex cabinet.
However, the dividing network affects characteristics of sound pressure frequency, phase and impedance.
FIG. 12 shows a mechanical equivalent circuit of the speaker system. Mechanical impedance Zm of the speaker system is connected to the compliance C0 of the cavity 1a and the mass m0 of the duct 3 in series, respectively. The compliance C0 and the mass m0 are connected in parallel with each other.
When a driving force F is produced in accordance with an input signal from an amplifier thorough a dividing network, a diaphragm of the speaker unit 2 is vibrated to emit sound of volume velocity U0. The volume velocity U0 is divided into volume velocity U1 and volume velocity U2 in dependency on the values of the compliance CO and the mass m0, respectively. The volume velocities U0 and the U2 are combined to the volume velocity U1.
FIG. 13 shows characteristics of sound pressures (SPL) which are obtained by converting the volume velocities U0, U2 and U which is a combination of U0 and U2. In the speaker unit, the dividing network is not provided.
FIG. 14 shows the combined sound pressure U when the dividing network is used. The dividing network operates to cut the high frequency range of the sound pressure U in accordance with a high-cut characteristic. Furthermore, the sound pressure is humped as shown by a dotted line at a frequency fp which is slightly higher than the lowest resonance frequency fo.
FIG. 15 shows the combined sound pressure U and an impedance characteristic Z. The dotted lines show the sound pressure and impedance characteristics without the dividing network.
FIG. 16 shows an equivalent circuit for explaining why the sound pressure is humped when the dividing network is used. Inductance L of low-pass filter in the dividing network is connected to a capacitive component Ce in series to compose a series resonance circuit. Since the impedance is reduced at the frequency fp, flow of the current is increased.
Accordingly, in the speaker system with the dividing network, the sound pressure is humped at the frequency fp so that the sound pressure of the low frequency range is masked, thereby producing an oppressive sound in low frequency range.
In order to eliminate the disadvantage, a correction circuit is provided between the dividing network and the speaker system to correct the hump of the sound pressure. However, since the correction circuit is costly, manufacturing cost of the speaker system is increased.
FIG. 17 shows a bass-reflex cabinet of another conventional speaker system without using the dividing network. In the system, the high frequency range is cut in accordance with compliance in a cavity of the cabinet in front of the speaker unit.
A bass-reflex cabinet 4 has a central partition 5 to divide the cabinet 4 into a front cavity 5a and a rear cavity 5b. The speaker unit 2 is secured to the partition 5 facing on the front cavity 5a. The front cavity 5a has compliance C1, and a duct 6 having a mass m3 is provided on a front panel 4a of the cabinet 4. The rear cavity 5b has the compliance C0, and a duct 7 having the mass m0 is provided on a rear panel 4b opposite to the duct 6.
FIG. 18 shows an equivalent circuit and FIG. 19 shows a sound pressure characteristic SPL and an impedance characteristic Z of the system of FIG. 17.
FIG. 20 shows a modification of the speaker system of FIG. 17. The speaker unit 2 is secured to an upper portion of the partition 5 and a duct 7' is provided on the partition 5 at a lower portion of the speaker unit 2 for communicating the rear cavity 5b with the front cavity 5a. The rear panel 4b has no duct. Other structures are the same as those of FIG. 17 and the same parts thereof are identified with the same reference numerals as FIG. 17.
FIG. 21 shows an equivalent circuit and FIG. 22 shows a sound pressure characteristic SPL and an impedance characteristic Z.
In these speaker system, the compliance C0 determined by the volume of the rear cavity 5b and the mass m0 of the duct 7 affect the low frequency characteristic. If the values of compliance C0 and mass m0 are increased, the low frequency characteristic is increased.
The compliance C1 determined by the volume of the front cavity 5a and the mass m1 of the duct 6 cause an unnecessary high frequency range to attenuate. In order to increase the low frequency characteristic, it is necessary to increase the compliance C0 and C1, causing the sizes (volumes) of the cavities 5a and 5b to be increased.
As shown in FIGS. 19 and 22, each of the sound pressures has a double humped characteristic. Thus, it is possible to obtain a flat top characteristic at a necessary frequency range.